chore: scaffold Bun + React + shadcn project

bun init --react=shadcn, with the demo API routes removed since this app
has no backend. Adds project invariants to CLAUDE.md and a smoke test
proving the domain layer imports with no DOM.

src/index.html

@@ -0,0 +1,12 @@
+<!doctype html>
+<html lang="en">
+  <head>
+    <meta charset="UTF-8" />
+    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
+    <title>Budget 2.0</title>
+    <script type="module" src="./frontend.tsx" async></script>
+  </head>
+  <body>
+    <div id="root"></div>
+  </body>
+</html>